1. Field of the Invention
The present invention relates to an apparatus and a method for smoothing a welded seam of a steel pipe. More particularly, the present invention relates to a method and apparatus for smoothing a welded seam of steel pipe by successively subjecting a steel strip in a welded pipe production line to cylindrical shaping with a forming roll to form an open pipe, and smoothing in the production line a thick walled portion of the pipe that has been pressure-welded in a proper temperature range of solid-phase pressure-welding.
2. Description of the Related Art
Welded steel pipes are produced by subjecting a steel sheet or a steel strip to cylindrical shaping and then to seam welding. Methods of producing such steel pipes can be roughly divided into electric resistance welding, forge welding, and electric arc welding according to outside diameters and uses.
Steel pipes having small to medium outside diameters are produced by an electric resistance welding method utilizing high-frequency induction heating. This welding method is devised to cylindrically form a steel strip with a forming roll into an open pipe that is then heated at ends of two opposite longitudinal edges by means of high-frequency induction heating at a temperature above the melting point of the steel. Those opposed end faces of the open pipe are subsequently butt-welded with a squeeze roll to form an electric resistance welded steel pipe. See, for example, Vol. 3 (3) pp. 1056 to 1092 of the third edition of Handbook of Steel.
One of the problems with this method is that when the opposite longitudinal edges of the open pipe are heated to a temperature higher than the melting point of the steel, molten steel flows under the influence of electromagnetic force forming an oxide that invades the welded seam. This has a tendency to or causes weld defects and molten steel splashes.
In order to overcome this problem, a method of producing an electric resistance welded steel pipe having two heaters is proposed in Japanese Unexamined Patent Publication No. 2-299782. A first heater heats opposite longitudinal edges of an open pipe to a temperature higher than the Curie point, and a second heater further heats the edges to a temperature higher than the melting point of the steel. Thereafter, two opposite longitudinal edges are butt-welded by a squeeze roll provided immediately downstream of the heaters to produce a steel pipe. In addition, Japanese Unexamined Patent Publication No. 2-299783 proposes an apparatus for producing an electric resistance welded steel pipe in which two opposite longitudinal edges of an open pipe are preheated with a current of a 45 to 250 kHz frequency applied by a first heater, and then two opposite longitudinal edges are further heated to a temperature higher than the melting point of the steel by a second heater and butt-welded with a squeeze roll.
These methods of producing electric resistance welded pipes teach heating two opposite longitudinal edges of the open pipe in a uniform manner, but the resulting flow of molten steel suffer may cause beads to form on inner and outer surfaces of the pipe during butt-welding because two opposite longitudinal edges of the open pipe are heated to a temperature higher than the melting point of the steel. The beads on the inner and outer surfaces should be removed after butt-welding. This removal is usually conducted by the use of a bead-cutting tool.
However, the bead-cutting tool causes additional problems. The time needed to replace the bead-cutting tool can be long due to adjustments in the amount to be cut, and wear or damage to the bead-cutting tool. This problem is especially severe when producing pipe at a high speed exceeding 100 m/min, which reduces the life of the bead-cutting tool and thus forces frequent replacement. For this reason, the pipe production line may be unproductive for prolonged periods.
Consequently, bead cutting imposes a bottleneck on production of welded steel pipes and prevents higher productivity.
On the other hand, a highly productive method of making a forge-welded steel pipe is also known to be suited for the formation of a steel pipe of a relatively small diameter. This method heats a successively supplied steel strip to a temperature about 1,300.degree. C. in a heating furnace and thereafter subjects the steel strip to cylindrical forming with a forming roll into an open pipe. High-pressure air is sprayed on two opposite longitudinal edges of the open pipe to descale the edges, and then oxygen is sprayed onto the edges with a welding horn. The temperature of the edges is increased to about 1,400.degree. C. by the oxidation heat and thereafter the edges are butt-welded and solid-phase welded by a forge welding roll to form a steel pipe. See, for example, Vol. III (3), pp. 1093 to 1109 of the third edition of Handbook of Steel.
However, this method is not without problems. Since the two opposite longitudinal edges of the open pipe surfaces are not sufficiently descaled, scales get into the butt-welded portion, and the strength of the seam is considerably inferior to that of the base material. For example, the electric resistance welded steel pipe achieves a flatness-height ratio h/D of 2t/D (with reference to FIG. 12, h is the height of the pipe when cracking occurs in the welded seam when the pipe is compressed and D is the outside diameter before compression, and where t is steel thickness), whereas the forge welded steel pipe can achieve a flatness-height ratio h/D of only about 0.5. In addition, the steel strip is heated to a high temperature, so that scales are produced on the surface of the pipe, thereby degrading the surface texture.
The forge welding method has a higher productivity than the electric resistance welding method due to its high pipe producing speed of 300 m/min or higher, but has poor seam quality and surface texture. For this reason, the forge welded steel pipe cannot be applied to a steel pipe requiring high strength reliability and surface quality, such as STK of JIS (Japanese Industrial Standards) or the like. In order to solve the above problems, the present inventors have devised a solid-phase pressure-welding pipe production method. In this method, two opposite longitudinal edges of the open pipe is subjected to induction heating (hereinafter, referred to as edges preheating) in the temperature range (hereinafter, referred as the preheating temperature range) higher than the Curie point (about 770.degree. C.) but below the melting point of the pipe. Then, a uniform temperature of two opposite longitudinal edges is ensured within the preheating temperature range by air cooling and thereafter two opposite longitudinal edges of the open pipe are pressure-welded by being subjected to the induction heating (hereinafter, referred to as the real heating) in a proper temperature range of solid-phase pressure-welding (1,300 to 1,500.degree. C.). The steel pipe produced by the solid-phase pressure-welding pipe production method requires no bead cutting unlike the conventional welded pipe, so that it can be produced by high pipe producing speed and has high productivity, and more over, causes no deterioration in the seam quality and surface texture due to oxidation. As shown in FIG. 11, however, a thick walled portion 6 that protrudes 5% or more of the thickness of the pipe 4 may be generated on a welded seam 5 of a solid-phase pressure-welded steel pipe 4 due to the temperature of the edges or degree of squeezing by the squeeze roll. Thick walled portion 6 is undesirable because it degrades the workability of the welded steel pipe, such as screw cutting, and promotes thickness deviation, such as inner surface angularity when squeeze-rolling the steel pipe.